Regarding its principal features, the stock feed at a paper machine is generally as follows. The stock components are stored at the paper mill in separate storage towers. From the storage towers, the stocks are fed into stock chests, and from the stock chests further into a common blend chest, in which the stock components are mixed with each other. From the blend chest, the stock is fed into a machine chest, and from the machine chest there is an overflow back into the blend chest.
From the machine chest, the stock is fed into a dilution part of the wire pit, in which the stock is diluted with white water recovered from the wire section and serving as dilution water. From the wire pit, the stock is fed through centrifugal cleaners into a deaeration tank. From the deaeration tank, stock free from air is fed through a machine screen into the headbox, i.e., into the inlet header thereof, and through the slice opening of the headbox to the wire section. A bypass flow of the headbox is fed back into the deaeration tank, and the white water recovered from the wire section is fed into the wire pit.
The basis weight and the ash content of the paper are measured on-line right before reeling from a ready, dry paper, usually by means of measurement apparatuses based on beta radiation and x-radiation. Based on this measurement, the basis weight of the paper is regulated, for example, by means of a so-called basis weight valve by whose means the stock flow after the machine chest is controlled. A second possibility is regulation of the speed of rotation of the pump that feeds stock from the machine chest into the wire pit. The ash content is controlled by dosing of fillers. The basis weight profile of the paper in the cross direction is obtained when the measurement apparatus is installed to move back and forth across the web.
In prior art arrangements of regulation in a paper machine, the metering of component stocks usually takes place with the aid of the surface level in the blend chest, the consistency of component stock, and a pre-determined stock proportion reference value. The ash contents of component stocks are not used for controlling the metering of component stocks. The measurement values obtained by means of measurement of basis weight are used in the control of the basis weight valve situated after the machine chest, but these values are not used for controlling the metering of component stocks.
In prior art regulation of basis weight, exclusively the overall consistency and the overall flow rate of the stock are controlled. Regulation of basis weight taking place by means of the flow departing from the machine chest is disturbed, among other things, by disturbance in the consistency of the machine stock, which disturbance arises from incomplete mixing of the stock in the blend chest and in the machine chest. The volumes of the blend chest and of the machine chest are considerable, so that the regulation of their surface levels readily starts oscillating, which results in disturbance in the regulation of the basis weight. From the recovery of fibers, ash disturbance arises in the blend chest. The dynamics of the recovery of fibers produce different dynamics and dwell times for a part of the stock. Owing to the large volumes of the machine chest and the blend chest, a long stilling time is required before the basis weight can be set at the desired level. For this reason, a change in the paper grade being produced by the machine is slow.